1. Field of Invention
Embodiments relate to optical waveguides, their fabrication, and applications. More particularly, embodiments relate to high-index-contrast and mechanically stable air-clad optical waveguides and deep-trench etching and wafer bonding methods for making said waveguides, and applications thereof
2. Background of Art
There are reported technologies for achieving air-clad waveguides in order to take advantage of the wide optical transparency window that it offers. In U.S. Pat. No. 7,920,770 B2, a general method for forming an air cladding below a waveguide is presented, but it does not offer a means for densely integrating devices of this sort, and it requires multiple lithography steps. In U.S. Patent Application No. 2013/0322811 A1, another form of waveguide suspended over an air trench is presented, though it still retains a lower silicon oxide cladding, preventing it from being useful in the mid-IR spectrum due to the optical absorption of silicon dioxide. It also requires precision bonding or additional alignment fabrication steps.
Another suspended membrane optical waveguide fabrication technique was demonstrated earlier in “Mid-Infrared Suspended Membrane Waveguide and Ring Resonator on Silicon-on-Insulator,” by Z. Cheng et al. and published by IEEE Photonics Journal, vol. 4, no. 5., pp. 1510-1519, October, 2012, in which the buried oxide layer was removed by selective wet etching underneath pre-fabricated silicon waveguides, resulting in a suspended membrane. This had the drawback of a large minimum width for the suspended area, and substantially reduced membrane strength due to the need to form “etch holes” on the surface of the silicon.
An improved idea was published by the inventor in “High-Contrast, All-Silicon Waveguiding Platform for Ultra-Broadband Mid-Infrared Photonics,” published by Applied Physics Letters, vol. 103, no. 15, p. 151106, October, 2013, which directly bonded a silicon membrane over a pre-fabricated air trench. However, it required a precision alignment step and etching of the membrane in order to form the waveguide, resulting in some weakening and constraining the maximum amount of etching before damaging the membranes.
An alternative type of air-clad waveguide was demonstrated in “Air-Clad Silicon Pedestal Structures for Broadband Mid-Infrared Microphotonics,” by P. T. Lin et al. and published by Optics Letters, vol. 38, no. 7, p. 1031, March, 2013, in which a rectangular silicon waveguide rests upon an etched silicon pedestal. However, this is not compatible with dense integration, and the strong lateral confinement of the waveguides and substantial separation between the structures implied by the fabrication method prevents evanescent coupling between adjacent modes, a key requirement for resonant structures on an integrated platform.
Therefore, there is a need for an integrated photonic platform that can operate over octave-spanning or even multi-octave spectral windows. Such systems require a suitable platform for fabrication which offers low propagation losses, tight optical confinement, and accurate control over dimensional characteristics. Normally, these requirements are at odds with each other, making it difficult to realize high-performance systems without compromising key features.